1. Field of the Invention
The invention relates to stator structures.
2. Description of the Related Art
FIG. 1 shows the structure of a conventional brushless direct current (DC) motor disclosed by U.S. Pat. No. 6,013,966. The brushless DC motor includes a stator structure.
The stator structure includes a top yoke 10 and a bottom yoke 20. A power coil is wound between the top yoke 10 and the bottom yoke 20. The stator structure is an axial stator structure. A plurality of salient poles 1 conducts corresponding poles to drive a rotor 2 to rotate when the power coil is supplied with current.
The brushless DC motor further includes two permanent magnets 3 disposed outside the rotor 2 for fixing the rotor 2 at a start-up position, thus providing an appropriate start-up torque.
To generate a sufficient start-up torque, two permanent magnets 3 are located at two fixed positions. The included angle between the stator and each permanent magnet 3 is precisely θ. To fix the rotor 2 at a start-up position utilizing the two permanent magnets 3, the rotor 2 is covered with a non-magnetic conductive material such as plastics.
However, the magnetic force between the stator and the rotor 2 is reduced by the block of the non-magnetic conductive material to influence, and a torque of the rotor 2 is influenced during rotation.